Field of the Invention
The present invention relates to a sheet conveyance apparatus for conveying sheets, and an image forming apparatus equipped with the sheet conveyance apparatus.
Description of the Related Art
In an image forming apparatus for forming images via an electrophotographic method according to the prior art, a toner image formed in an image forming portion is transferred to a sheet supplied from a feeding portion, and then the sheet is guided to a fixing unit where unfixed toner on the sheet is fixed to the sheet. A sheet processing apparatus is connected to such image forming apparatus for processing the sheet on which the image has been formed, such as sorting, stapling, punching and the like.
When fixing the toner image on the sheet by applying heat and pressure to the sheet via the fixing unit, the sheet may be curled (deformed). When the sheet is deformed, jamming of the sheet may occur in the sheet processing apparatus, or processing accuracy such as stacking performance or aligning performance may be affected. Therefore, the curling of the sheet must be corrected in order to improve the quality of the processed sheet.
In the prior art, a curl correcting apparatus is known where a curled sheet is subjected to curling in the opposite direction as the formed curl. One example of such curl correcting apparatus is equipped with a first curl correcting portion where a curl curved upward in a convex is corrected, and a second curl correcting portion where a curl curved downward in a convex is corrected. Then, according to the direction of the curl, the sheet having been curled is corrected using at least either one of the first curl correcting portion and the second curl correcting portion.
The curling of a sheet occurs, for example, by the toner transferred onto a sheet being melted by the heat applied in a fixing unit, and then being condensed by cooling. The size of the curl on the sheet varies, depending on the type of the sheet, temperature and humidity, image density, and so on. Therefore, when correcting the curl using the curl correcting apparatus, an amount of correction of curl by the first and second curl correcting portions is determined based, for example, on the temperature and humidity, a moisture content of the sheet, the sheet type, sheet thickness, the image density and so on.
Each curl correcting portion of the curl correcting apparatus is normally equipped with a hard roller for conveying a sheet and a soft roller pressed against the hard roller. Then, when pressing the soft roller against the hard roller, a nip portion being curved, by the hard roller penetrating the soft roller, is formed, and the curled sheet is passed through the curved nip portion to correct the curl.
Now, when correcting the curl, a pressing force of the hard roller against the soft roller is changed to thereby change an amount of insertion (amount of penetration) of the hard roller to the soft roller, by which the amount of correction of the curl can be adjusted. The amount of insertion (amount of penetration) is changed as described, wherein Japanese Patent Application Laid-Open Publication No. 9-30712 provides a curl correcting portion using a motor and a cam member to change the position of one of the rollers.
However, when changing the amount of penetration (pressing force) of a roller pair by the cam member, an area on an upward slope side of the cam member will not be an issue, but a force in a same direction as a direction of rotation acts on a downward slope-side area of the cam member from a cam follower. In that case, a rotation speed of the cam member will exceed a rotation speed being rotated by the motor, and thus, an amount of rotation of the cam member is increased and an adjustment position of the cam member is displaced.
And as a result, the position of the roller pair may be displaced from a given position. Especially in the curl correcting apparatus where a large pressing force is applied to the roller pair compared to a normal conveyance roller for conveying the sheet, the rotation force acting on the cam member from the cam follower is increased, and the problem becomes more significant.
Further according to the curl correcting apparatus, the position of the hard roller with respect to the soft roller must be changed to a plurality of locations, and positional accuracy of the hard roller at each position becomes important. When the cam member is displaced, the curl correction amount will be dispersed, and insufficient correction or excessive correction may occur. Furthermore, the position of the roller pair is not only displaced from the given position, but a negative torque occurs to the motor, so that step-out of the motor may occur.
Therefore, in a sheet conveyance apparatus taught in Japanese Patent Application Laid-Open Publication No. 2004-59237, a brake panel is pressed against an outer circumferential surface of the cam member within a given range of angle of rotation of the cam member, by which a brake force is applied to the rotation of the cam member, and the rotation of the cam member.
However, by regulating the rotation of the cam member by pressing a brake panel against the outer circumferential surface of the cam member as described, friction force may be varied by the chipping of the brake panel or the deterioration of surface durability, and the brake force. That is, the regulating force regulating the rotation of the cam member, is varied. Furthermore, the slidability between the cam member and the brake panel is changed and the load may be varied drastically, and in that case, step-out of the motor or noise may occur.